Validity and reliability of a finger training tool for assessing metacarpal phalangeal joint ranges of motion in asymptomatic participants

This pilot study aims to evaluate concurrent validity using the goniometer as a reference tool and test–retest reliability of flexion of metacarpal phalangeal joint (MCP) measurements taken from a finger training device (air-guitar system) in healthy participants. There were ten self -reported asymptomatic participants recruited to test the devices. The measurements of all metacarpophangeal joints of the dominant hands were conducted using a finger goniometer and the air-guitar system. Two measuring sessions were conducted on the same day. The concurrent validity of the air-guitar indicated by strong concordance correlation coefficient (0.62–0.90) with the goniometer and mean difference (approximately 1°) between the two instruments are well below the limit of 5°. The test–retest reliability of MCP measurements from the air-guitar glove (0.82–0.99) was acceptable as a clinically meaningful measurement tool as the intraclass correlation coefficients were higher than 0.7. The standard error of measurement and minimal detectable change of the air-guitar are similar to those of the goniometer. The air-guitar tracking features, when used as a home-based therapy tool, may assist in monitoring change of MCP flexion over a time course with good reliability and strongly associated with the measurements from the goniometer.


Device design: air-guitar system
The air-guitar system focuses on finger range of motion exercises with adjustable resistance to facilitate hand rehabilitation (Fig. 1).The system comprises a joint-monitoring glove with adjustable resistance for muscle strengthening, a USB-controller, a serious game with grip exercises and instruction available in English and Thai, and specifically designed therapeutic music for motivation.The glove monitors the movement of metacarpophalangeal joints (MCP), which are crucial for grip and grasp functions 20 .The software features an assessment room and a therapy game room (Fig. 1).In the assessment room, the air-guitar hand device records the static active range of motion, much like a finger goniometer does in clinical practice.The therapy room provides a music game featuring motivational music therapy where the user can practice gripping in response to computergenerated visual cues on the screen.The system would provide a real-time score for timely and correctly completed movements it recognized.A score and MCP range of motion for each finger are reported at the end of the training session to support specific treatment plans and track progress.Therefore, it is important to evaluate the concurrent validity and the test-retest reliability of the system.The standard goniometer technique in a clinical setting is regarded as reference and comparison with previous literature.

Equipment
Goniometer A stainless-steel Jamar finger goniometer (JAMAR®, Patterson Medical Supply, CA) equipped with both long and short arms, allows for precise measurement of finger movement with an accuracy of up to five degrees.Measurements ranged from 120° of flexion to 30° of hyperextension (Fig. 2).

Air-guitar system
The "Air-guitar system" is a home-based finger training apparatus comprising a plastic wrist-hand orthosis with individual finger extensions and integrated software for serious gaming and measurement of MCP joints (Fig. 1).Developed at Chiang Mai University in Chiang Mai, Thailand, this system is currently in the pre-clinical stage of development.The device has successfully met electrical safety standards for medical devices and has undergone rigorous usability and medical software testing.
The electronic circuit in the monitoring system is designed to connect the ESP32 microcontroller to the potentiometer 50 k plate sensors, as shown in Fig. 3a and b, respectively.The potentiometer functions as a variable resistor, changing resistance in accordance with the wiper position.Five potentiometers were used to place the angle sensors on each finger's MCP.When a voltage (V_in) is applied, the position of the wiper affects the output voltage (V_out), which is proportionate to its location along the resistive track.Thus, the voltages determined the angles, as seen by the relationship between the angle and V_out in Fig. 3c.The air-guitar system is capable of recording angles between 0 and 135 degrees due to its design.
The plastic wrist-hand orthosis is available in three sizes-small, medium, and large-based on palm width.After selecting the appropriate size, participants wear the air-guitar device, as shown in Fig. 4. Soft Velcro straps secure the plastic structures to each finger, enabling independent finger movement.During the initial calibration process, the participant places his hand and fingers flat on a table to establish a neutral alignment of zero.Each user only needs to perform this calibration once, which facilitates ongoing patient monitoring at home.
The air-guitar device features two primary modes: the assessment mode (Fig. 4a), which measures the flexion range of the MCP joints, and the game mode (Fig. 4b), which promotes intensive finger-grip exercises through serious music games.Both modes record finger motion data for continuous comparison throughout rehabilitation.This study used only the assessment mode to evaluate the MCP joints' active maximal flexion.www.nature.com/scientificreports/Protocol After informed consent was obtained, the participant was brought to the motion capture room at the physiotherapy department at Chiang Mai University.A computer-generated order randomly assigned the participant to either the air-guitar or the goniometer group.After finishing the first session, the participant had an approximately 15-min break before starting the second session.
A clinician with 10 years of clinical experience performed the goniometric measurements, while a technician supported the use of the air-guitar system throughout the sessions.To ensure that the clinician and technician were blinded to each other's readings, the clinician and the technician performed measurements independently.The technician fitted the air-guitar device on the patient, ensuring proper connectivity and adherence to the user manual.The technician assisted the participant in following the on-screen instructions, verified the device's fit, recorded the measurements displayed on the screen, and documented them on the appropriate form.Additionally, the technician addressed any concerns regarding the air-guitar system, including any discomfort experienced by the participant.

MCP joint measurements
For the goniometer group, the participant had his forearm in a neutral position and had his elbows on the table.The recommendation of the American Society for hand therapists for the finger joints was followed 22 .The clinician positioned the goniometer on the dorsal aspect and examined five MCP joints.The participant was instructed to flex their MCP joint separately, one at a time, and then the clinician read the measurement on the goniometer with 5° increment.If the goniometer indicated a space between the increments, 2.5° was read.After the measurement was taken, the participant was instructed to fully extend his MCP joint before fully flexing for next measurement.These steps were repeated for three measurements before proceeding to assess the other joints.
Using the air-guitar device, the active range of MCP joint flexion was also determined.The participant wearing the air-guitar device received assistance from the technician.The participant was given instructions to do the measurements as shown on the computer screen in the "Assessment room" mode.The technician recorded the readings on the screen.
All measurements were performed three times, consistent with clinical routine, by the same examiners.For further analysis, the mean value of these measurements was used.All measuring procedures were documented on a blank case record form to minimize the influence of previous readings.Measurements were repeated in the afternoon under identical conditions to evaluate the concurrent validity and test-retest reliability of the methods.

Concurrent validity
The concurrent validity indicates the degree to which scores on the measure agree with scores on another measure taken at the same time.In this context, Lin's concordance correlation coefficients (CCC) are calculated as CCC is the concordance between two readings: a new test or measurement and a gold standard test or measurement.This statistic evaluates the agreement between two methods measuring the same variable by measuring the variance from the 45° line across the origin (the concordance line) 23 .It can be used in small sample sizes of as few as ten.Bland and Altman plot is also presented for visualization with Mean difference and Limit of agreement between two methods.This information is helpful for clinical application.In this study, the concurrent validity was determined between the goniometer and the air-guitar device for maximum flexion position of the MCP joint using CCC and the Bland Altman plot.The acceptable mean difference between two-method is set at 5° as this is an approximate variability of the standard goniometer readings of the joint angle measurements in the hand and the goniometer used can read precisely with 5°.This mean difference is also comparable to previous report 24 .The interpretation of CCC is still controversial.Akoglu suggested that CCC should be interpreted closely compared to other correlation coefficients, like Pearson's correlation coefficient: 1 indicates perfect agreement, 0.8-0.9indicates very strong agreement, 0.6-0.7 indicates moderate, 0.3-0.5 indicates fair, 0.1-0.2indicates poor and 0 indicates non agreement 25,26 .
Mean difference and 95% limit of agreement were also calculated for Bland and Altman plot to visualize data 27 .All data points (6 pairs of data) from each participant were plotted to maximize the number of pairs of data to visualize any possible relationship between mean differences and the best possible true value.The differences between the goniometer and the air-guitar measurements were plotted against the mean of the two measurements to investigate any possible relationship between measurement error and the true value.The mean of the two measurements is the best estimate for the true value 28 .The use of a standard or reference method instead of the mean of the two measurements is controversial because a relation between difference and magnitude is likely, though there is none 29 .The regression lines and confidence interval limits were also presented to visualize the trend of proportional difference 30 .

Test-retest reliability
Test-retest reliability indicates the size of measurement error between test and retest sessions.In addition to Intraclass correlation coefficient (ICC), the minimal detectable change (MDC) and standard error of measurement (SEM) are calculated.These parameters facilitate comparison and interpretation of data.Test-retest reliability of the numeric data was assessed by using the Intraclass Correlation Coefficient [ICC(1,1)], one-way random effects, absolute agreement, single rater/measurement.The values were classified as follows: < 0.5 indicate poor, 0.50-0.75indicates moderate, 0.75-0.9indicates good reliability, and 0.90 indicates excellent reliability 31 .The acceptable ICC values need to be equal to or greater than 0.70 for a clinically meaningful measurement 32 .
SEM indicates that the true value lies within ± 1SEM of measurement with 68% confidence and ± 2SEM with 95%.A limit of 5° of measurement error is set based on previous literature 24,33 and (SEM) was calculated using SEM = SDx √ 1 − ICC where ICC was from previous calculation 24 .In order to account for the extent of change that is not the consequence of random variation or measurement error, the minimum detectable change at the 95% confidence level (MDC 95 ) was calculated using MDC 95 = (2) × 1.96 × SEM 34 .
The goniometer, as a standard clinical measuring tool of angular joint position, can only measure the static angular position of the MCP joints.Therefore, only maximal flexion of the MCP joints will be evaluated in this paper as it reflects the functional range of motion and affects the size of object the hand can hold.

Results
Ten healthy right-handed participants with a mean age of 34.6 years (eight men and two women) were evaluated with the air-guitar system and a finger goniometer (Table 1).

Concurrent validity
According to Table 2, CCC of MCP3 is 0.62, indicating moderate agreement, while other MCP joints range from 0.8 to 0.9 indicating very strong agreement between measurements between the goniometer and the air-guitar www.nature.com/scientificreports/system.The mean differences (close to zero) are less than 5°of the acceptable value priori set.The Bland-Altman plots show a graphical presentation of all 60 pairs of measurements taken from two sessions (Fig. 5).The zero line lies within the range of differences between the goniometer and the air-guitar system.The limit of agreement is ranging between 6 and 11° across MCP joints.Though the regression lines show small negative trend along the graph indicating small proportional error (Supplementary Table S1).However, a negative trend is expected when readings are close to the upper limit of 90°.The differences are very small within the limit of agreement.There seem to be no tendency of bias across these joints.

Test-retest reliability
The maximal flexion of MCP joints [1][2][3][4][5] were reported in Table 3. MCP angles between two methods were comparable.All ICC values from the goniometer and from the air-guitar are indicative of good to excellent reliability and well above the acceptable level of 0.7 (Table 4).However, the interval estimates (95% CI) of the air-guitar readings seem to be lower than those of the goniometer.Most MDC 95 and SEM from the air-guitar measurements seem to be similar to the goniometer measurements.MDC 95 and SEM from MCP1 and MCP2 appear to be slightly greater than those of others.

Concurrent validity
In this study, the goniometer is used as a reference method to demonstrate the validity of measurements of active flexion from the air-guitar for MCPs.Image techniques such as X-rays may provide more accurate angles of joints.However, the cost and risk may be inappropriate for this study, and the goniometer is a current clinical standard practice to monitor angles of joints.Therefore, the result of this study could facilitate the current clinical practice.The CCC was selected to demonstrate the concurrent validity of MCP measurements between the goniometer and the air-guitar systems, yielding CCC values ranging from 0.62 to 0.90.This is considered very strong 26 .However, McBride's criteria suggest that a CCC value of less than 0.90 indicates poor agreement, 0.90-0.95indicates moderate agreement, 0.95-0.99indicates substantial agreement, and greater than 0.99 indicates almost perfect agreement.By McBride's criteria 35 , the results might be deemed very poor since the readings were not identical.However, McBride's criteria may not be entirely appropriate for this study due to the differing resolutions of the devices: the goniometer has a resolution of 5°, whereas the air-guitar system has a resolution of 0.01°.This discrepancy in resolution likely accounts for the observed differences between the two methods.
The mean differences between the two methods (approximately 1°) are well within the acceptable range (5°), supporting the validity of the air-guitar system for assessing active flexion of the MCP joints.However, the LOA ranges from 6° to 11°, suggesting that caution should be exercised when interpreting data obtained with the airguitar system.This wide LOA range may be attributed to the differing resolutions of the devices, as previously mentioned.Additionally, measurements were taken at the end of active flexion, and despite identical verbal instructions, some variations within sessions may have occurred.Our results suggest that there is an absence of systematic errors as the mean differences between the two measurements are consistently close to zero and within the limits of agreement.As well as the proportional errors indicated by the regression lines and the confidence interval limits are relatively small within the limit of agreement.Moreover, there is no tendency for one measurement to be consistently higher or lower than the other, across different values of readings.Although the measurements from the two methods are not identical, the results indicate a very strong agreement between them.While the air-guitar system may not be a suitable replacement for the goniometer in clinical settings, it serves as a useful device for monitoring the active maximal flexion of the MCP joints during home-based training.Future research should focus on validating the dynamic measurements of the air-guitar device against gold standards, such as 3D motion analysis systems, to accurately quantify dynamic data.

Test-retest reliability
The test-retest reliability of MCP flexion measurements using the air-guitar system proved acceptable for clinical use, with ICCs exceeding 0.7 32 .Both measurement methods demonstrated similarly high ICCs, with the air-guitar system showing smaller CIs for MCP1 and MCP3-MCP5, indicating less variability compared to the goniometer.This may be because the resolution of the Air-guitar (0.01°) is finer than that of the Jamar finger goniometer (5°), and the use of the device may reduce some human errors during test-retest measurements.The current goniometer technique involves several placements of the flat goniometer arms on the fingers and hand surfaces, which are not completely flattened.This difficulty is similar to the wrist, as suggested by Armstrong et al. 36 .Also, these two joints (MCP4-5) are relatively difficult to approach manually.In contrast, the goniometer measurement of the MCP1 showed slightly higher ICC and narrower range of CI than those from the air-guitar system, this may be because of stabilization of the surrounding joints.The manual approach required the examiner to stabilize the carpometacarpal joints while taking measurements, while the air-guitar system did not.Moreover, with the goniometer and therapist's hand, the movements in the different planes were well stabilized.Whereas the patient actively moved without stabilization when using the air-guitar system.The test-retest reliability of the air-guitar is comparable to that of the goniometer and likely to be able to detect changes of MCP flexion over time course with caution of the MCP1-2 joints.
The SEM and MDC 95 calculated in this study were well below the 5° limit established by clinical routine measurements using a goniometer.The SEM and MDC 95 reflect measurement errors in the same units as the measurements themselves and are valuable for monitoring progress over time.SEM values for all joints measured by the air-guitar system ranged between 1° and 3°, similar to those from the goniometer.In our data, the MDC 95 for the air-guitar system ranged from 1° to 9°, and for the goniometer from 3° to 10°, indicating comparable results.This suggests that the air-guitar system is as capable of detecting changes in flexion as the goniometer.
Regarding goniometer measurements, the goniometer readings across all MCPs were roughly the same range of the previous reports.The MDC 95 range of 3°-10° were between 19° and 24°as previously reported by Reissner et al. and the SEM range of 1°-4° were between 7° and 9° reported by Reissner et al. 24 .The ICC from our goniometer measurements ranging from 0.75 to 0.97 was also within the ranges (0.59-0.86) 24 and (0.68-0.93) 37 , as previously reported.These ensure the goniometer data quality in this protocol is comparable to the others.

Limitation
This study is done as a preliminary study in a small sample size to demonstrate the concurrent validity and test-retest reliability of the hand therapeutic device for measurement of the maximal MCP flexion, approximately 90°3 8 , as a monitoring device for a home-based therapeutic exercise.The air-guitar system is validated using the clinical finger goniometer which is usually small with increments of 5°.Compared with 0.01° increment of the air-guitar, 5° is large.However, since the goniometer is the gold standard for measuring range of motion in clinical practice, the air-guitar is compared to it.
Blinding of measurements between morning and afternoon sessions for the same device could not be implemented, as the same clinician and technician conducted both sessions to ensure consistent measuring conditions.To minimize potential bias, measurements were recorded on separate blank case record forms, preventing the clinician and reader from seeing previous readings.Additionally, each participant had 15 measurements, and with two to three participants per morning session, it was unlikely that the clinician and technicians would remember previous readings and influence the results.The concurrent validity and test-retest reliability shown in this study seem to be acceptable for monitoring joint range in healthy participants.However, the minimal clinical change in different patient groups where the device may be clinically used should be further investigated to facilitate clinical application.In addition, the concurrent validity and test-retest reliability are limited to static maximal flexion measurements only.The maximal flexion may strongly associate with the functional flexion [MCP is ranging from 19 to 71°3 9 ] so it can only be used with caution.The functional flexion of the hand in different postures and movements can be measured using the air-guitar system.To quantify these dynamic movements and ensure certainty of use, it requires further investigation against 3D motion analysis as a gold standard.

Conclusion
As a home-based therapeutic device, the air-guitar monitoring characteristics could facilitate monitoring of MCP change over a time course with acceptable test-retest reliability as a clinically meaningful measurement tool and comparable to the goniometer with less than 5° difference.

Fig. 5 .
Fig. 5. 60 paired differences between flexion range of metacarpophalangeal (MCP) joints measured by goniometer and by air-guitar device.Solid line represents mean; upper dashed line shows the mean ± 1.96 standard deviation (SD) and lower dashed line shows the mean − 1.96SD.The long dash lines show the regression lines with gray continuous lines showing confidence interval limits.
Data processing and analysisAll analyses were conducted using STATA/MP17 (StataCorp, 4905 Lakeway Drive, College Station, TX 77,845, USA).Descriptive data: age, weight, height, Body Mass Index (BMI) and angles are reported as mean and standard deviation (SD) with minimal and maximal values.

Table 1 .
Demographic data of asymptomatic participants recruited in the study.N = 10, Male = 8 min-max.BMI, Body mass index.

Table 2 .
Validity parameters between the goniometer and the air-guitar system in the same sessions.MCPmetacarpophalangeal joint.

Table 3 .
Descriptive statistics for test -retest measurements from the goniometer and the air-guitar system.MCP, metacarpophalangeal joint; SD, standard deviation.

Table 4 .
Test-retest reliability of the goniometer and the air-guitar system.MCP, metacarpophalangeal joint; ICC, intraclass correlation coefficient; CI, confidence interval; SEM, standard error of measurement; MDC 95 minimum detectable change at the 95% confidence level.